#include<bits/stdc++.h>
#include<bits/extc++.h>
#define int long long
#define inf 0x3f3f3f3f3f3f3f3f
#define maxn 1234567
#define eps 1e-7
#define mod 1000000007
#define Mod 998244353
#define f(i,a,b) for(int i=a;i<=b;i++)
#define r(i,a,b) for(int i=a;i>=b;i--) 
#define fx(i,a,b,x) for(int i=a;i<=b;i+=x)
#define rx(i,a,b,x) for(int i=a;i>=b;i-=x) 
using namespace std;
using namespace __gnu_cxx;
using namespace __gnu_pbds;
int T;
int n,m;
void solve(){
	scanf("%lld%lld",&n,&m);
	if(m>=n)printf("1\n");
	else if(m<n/2)printf("-1\n");
	else{
		if(n&1)printf("-1\n");
		else printf("2\n");
	}
}
signed main(){
	T=1;
//	scanf("%lld",&T);
	while(T--)solve();
	return 0;
}